This application refers to a system and method for the elimination of a failure mode that can occur in transmission systems of the type described in WO 2004/099654, WO 2005/005868, WO 2005/005869, WO 2005/024261 and WO 2005/026570. However, similar problems may occur in other types of transmission systems.
The known systems disclosed in the above-mentioned publications are examples of instantaneous transmission systems. The known transmissions have a plurality of gear trains for transmitting drive between a transmission input shaft and a transmission output shaft. For a first gear train, a first gear wheel is rotatably mounted on one of a transmission input shaft and an output shaft and a second gear wheel is fixed to the other shaft, in mesh with the first gear wheel. A second gear train comprising third and fourth gear wheels is similarly arranged. The transmission also includes at least one gear selector mechanism that is located between the rotatably mounted gear wheels that is arranged to selectively lock them for rotation with the shaft on which they are mounted. When a gear wheel from a gear train is locked for rotation with the shaft, drive is transmitted between the input and output shafts via that gear train.
The arrangement is such that when drive is transmitted between the input and output shafts via one of the gear trains, for example the first gear train, the gear selector mechanism can select the new (second) gear train under power without disengaging the first gear train, by locking the rotatably mounted gear wheel of the second gear train to its shaft. Thus momentarily, drive is transmitted between the input and output shaft via two gear trains. The new gear train then overdrives the first gear train and the selector mechanism disengages the first gear wheel. Drive is then transmitted between the input and output shafts via the new gear train only, thus providing uninterrupted power through a gearshift. The selector mechanism is arranged such that the gearshifts can take place under acceleration or deceleration.
The gear selector mechanisms of the known transmissions have four modes with respect to each adjacent gear train:                Fully engaged in both torque directions (fully in gear);        Disengaged in both torque directions (neutral);        Engaged in the forward torque direction while disengaged in the reverse torque direction;        Disengaged in the forward toque direction while engaged in the reverse torque direction.        
The last two of the above four modes enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without torque interruption.
There is an inherent failure mode in any gearbox with more that two gear trains that has both of the last two modes. Thus it is possible in instantaneous transmission systems having at least three gear trains for two gears to be engaged simultaneously with opposing modes under some conditions, which causes the transmission to lock up. One of the most dangerous scenarios is if the direction of torque changes during a shift. If torque has a constant known direction during a shift, the natural sequence of events prevents the above failure mode. During a sudden reversal of the direction of torque immediately prior to, or during a shift, there is potential for the above failure mode to occur.
A transmission system typically includes at least three gear trains, and is likely to include four to six gear trains. A transmission having four gear trains requires two instantaneous gear selector mechanisms. The first gear selector mechanism is arranged to selectively engage the first and second gear trains and the second gear selector mechanism is arranged to selectively engage the third and fourth gear trains. Each gear selector mechanism includes first and second sets of engagement members having opposing ends with fixed opposing directions of torque transfer. This provides an inherent fail-safe arrangement against the above mentioned failure mode where the shift is from a gear on one side of the hub (selector mechanism) to a gear on the other side of the same hub, for example when the first selector mechanism selects between the first and second gears or when the second selector mechanism selects between the third and fourth gears.
The above failure mode can only occur if a gearshift is from a gear that is engageable by one of the gear selector mechanisms to a gear that is engageable by the other gear selector mechanism, for example when changing between second and third gears in the four speed transmission mentioned above, since this requires movement of both the first and second gear selector mechanisms. The first gear selector mechanism has to move out of engagement with the second gear train and the second gear selector mechanism has to move into engagement with the third gear train. If a torque reversal occurs when the second gear is still engaged by the first selector mechanism and the third gear is engaged by the second selector mechanism, the transmission may lock up.
The transmission described in PCT/GB2006/000743 addresses the above-mentioned problems by using a layout that is inherently safe. The layout ensures that each gear change takes place across the hub of a single gear selector devices, which is inherently safe. This is achieved by including a gear train that is arranged to be selected by the first and second instantaneous gear selector devices and by alternating subsequent gear selector devices on the input and output shafts of the transmission.
WO 2005/0058648 describes an electronic control system for measuring the direction of torque in the gearbox and managing some shifts such as a kick-down shift. By measuring the magnitude and direction of torque on all shifts it is possible to prevent gearbox lock up due to conflicting modes being engaged in two gears at once. However, control systems are complex and may introduce new failure modes into the transmission system. Since the control system does not affect the relative positions of the gear trains and the selector mechanisms in the instantaneous transmissions described above, the transmission layout remains inherently prone to the failure modes mentioned above should a problem occur with the control system.
United Kingdom patent application number GB0504628.9 teaches a transmission system that addresses the above problems by designing out the possibility of catastrophic gearshifts from taking place. A layout is used for a sequential transmission that alternates the gearshifts between instantaneous selector mechanisms located on different shafts.
Accordingly the present invention seeks to provide an improved transmission system that mitigates at least some of the aforementioned problems or at least provides an alternative solution to those problems.